The present invention relates generally to antennas, and more specifically the invention pertains to a microstrip patch antenna for radiating an electromagnetic signal with a polarization that is magnetically varied by an application of an in-plane magnetic field.
The term "polarization" refers to the process of making light or other radiation vibrate perpendicular to the ray. The vibrations are straight lines, circles or ellipses--giving plane, circular, or elliptical polarization, respectively.
The direction of polarization of an antenna is defined as the direction of the electric field vector. Most radar antennas are linearly polarized; that is, the direction of the electric field vector is either vertical or horizontal. The polarization may also be elliptical or circular. Elliptical polarization may be considered as the combination of two linearly polarized waves of the same frequency, traveling in the same direction, which are perpendicular to each other in space. The relative amplitudes of the two waves and the phase relationship between them can assume any values. If the amplitudes of the two waves are equal, and if they are 90.degree. out of (time) phase, the polarization is circular. Circular polarization and linear polarization are special cases of elliptical polarization.
Existing microstrip patch antennas are typically fabricated on dielectric substrates only, are very narrowband and radiate linear or circular polarization only, and are not frequency tunable. Achieving circular polarization requires an offset feed, or two separate feeds and a signal splitting network to provide the proper phase and magnitude signals to the feeds.
Investigations have been made of patch antennas fabricated on bulk ferrite substrates, and frequency tuning of a linearly polarized antenna, via the application of an in-plane magnetic field, has been demonstrated. The invention differs from previous work in that it does not require a ferrite substrate, but just a thin ferrite film and a dielectric substrate, and it radiates two orthogonal polarizations, and the phase relationship between these polarizations is varied by the application of an in-plane magnetic field. The task of providing a microstrip patch radiating element whose radiation polarization can be varied via the application of an in-plane magnetic field is alleviated to some extent by the system described in the following U.S. Patents, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,879,562 issued to Stem et al;
U.S. Pat. No. 4,821,041 issued to Evans;
U.S. Pat. No. 4,783,661 issued to Smith;
U.S. Pat. No. 4,780,724 issued to Sharma et al;
U.S. Pat. No. 4,660,048 issued to Doyle and
U.S. Pat. No. 3,811,128 issued to Munson.
The patents identified above, relate to various patch antenna designs. In particular, the Evans patent describes a microwave patch antenna which comprises a substrate of high dielectric constant, an aperture in the substrate and a patch conductor. The patch conductor is positioned on one major surface of the substrate and juxtaposed over the aperture. A ground plane is positioned on another surface of the substrate, and has an aperture juxtaposed to at least a substantial proportion of the patch conductor. A conductive cavity is RF-coupled to the ground plane at the aperture, and the length of the cavity is adjustable to tune the antenna.
The Smith patent is directed to a circularly polarized antenna which comprises first and second multiple patch antenna structures dimensioned to operate at two distinct frequencies. Each antenna structure consists of four shorted patches. The patches of the first structure are spaced from a ground plane by dielectric material, as are the patches of the second structure spaced from the patches of the first structure. The patches of both structures are disposed in the planes of the patches so that the radiation edges of the two patch structures form superimposed antenna structures.
The Sharma et al patent relates to a patch antenna with an internal tuning element. The patch antenna is formed on one broad surface of a semiconductor plate, and a ground plane is formed on a second broad surface. The semiconductor is doped in regions near a periphery of the patch to define a semiconductor junction. The junction has capacitance which tunes the patch antenna. The characteristics of the junction are controlled by bias to selectively tune the patch antenna.
The Doyle patent describes a microstrip patch antenna comprised of either a single element or a plurality of stacked antenna elements. The stacked elements have one or more feedpins connected to a corresponding number of conductive elements which are capacitively coupled to the antenna elements. The feedpins have an inductive reactance which is cancelled by trimmed flags to provide the capacitance necessary to cancel the inductance for tuning the antenna. Although these patents relate to various designs for patch antennas, they do not describe a patch antenna with radiation polarization that can be magnetically varied by the application of an in-plane magnetic field.